This invention relates to a method and an apparatus for preventing the formation of macro-segregations in continuous metal casting, and particularly carbide segregations.
In the continuous casting of high-carbon steels, for example ball bearing steels, high-speed tool steels and also other steels with high carbon content, distinctive carbide segregations appear which render the material unsuitable for many fields of application. The same kind of carbide segregations also can arise when the aforesaid steels are cast in conventional molds and during ESR-remelting at high melting rates.
Carbide segregations are formed during the solidification of the inner parts of an ingot. Due to the large solidification intervals of the steels, relatively thick zones of semi-solidified material are formed therein. In said zones dendrites form a porous network of solidified metal with a lower than average content of impurities in the material. Residual molten metal with higher carbon content is located in the intermediate spaces between the dendrites. During the solidification, the metal shrinks, partly as solidification shrinkage of about 4% and partly as cooling shrinkage in metal already solidified.
The metal or material solidifies from the outer surfaces inward to the center of the material. This results in several solidification front existing during the solidification process which grow toward the material center. In continuous casting, furthermore, a strand with unsolidified material in the center moves from a mold downward. Depending on the dimensions of the strand and its casting rate, the solidification zone, i.e., the zone within which semi-solidified material is present, varies in the longitudinal direction of the strand with respect to length and other dimensions. When the solidification zone has an unfavorable configuration, i.e. when it is long and thick, high stresses arise between the solidification fronts of the solid and semi-solidified zones. These stresses arise due to a difference in the cooling rate between the shell surfaces and the interior solid dendrite phase. As the material solidifies the solidification front is moved towards the center of the strand. Hereby, the temperature decreases when the solidification front passes. This decrease is often higher than the temperature decrease at the outer surface. This is the case when the solidification fronts meet in the center.
Thus, the stresses arise due to a temperature difference between the solid surface and the solid interior phase. As a result of these stresses, the fronts separate. The shrinkage gives rise to a pressure differential, which sucks down the melt through the porous semi-solidified material. This melt is enriched with impurities and alloying elements and, consequently, macro-carbide segregations are formed in the center of the strand. Corresponding macro-segregation conditions prevail for all alloys with large solidification intervals and give rise to segregations. These macro-segregations occur with respect to all alloying and non-metallic elements present in molten metals.
When the solidification zone is long, the metal solidifies and shrinks in the central portions of the strand, relatively large amounts of melt must be transported to the semi-solified zone. As a result thereof, substantial macro-segregations arise which form pores and cracks in the central portion. In continuous casting it is also known that carbide segregations can be reduced by carrying out the casting very slowly. The casting rate, however, in that case must be reduced so much that the process is uneconomical.
A process for controlling continuous casting against the formation of center segregation and center porosity is described in U.S. Pat. No. 3,974,559 to Kawawa et al. According to this process, a continuous casting is formed in a mold 11 and is then passed through a secondary cooling zone 12 in order to form a thick solidification shell on the strand. Thereafter, a series of reducing rolls are positioned at the front portion of the crater end within the strand in order to check the movement of the molten steel which contains impurities concentrated by reasons of the previous solidification within the secondary cooling zone. This patent discloses and claims that the reduction rate should be less than 1.5% for each pair of the reducing rolls in order to avoid center cracks which can be formed by too great a reduction in the slab thickness. This patent does not recognize the problem of macro-segregations across the ingot cross-section and along its length during a continuous casting operation and the related problems caused by the concentrated impurities within the liquid phase. Also, the disclosure of this patent does not take into the account the effects of the cooling shrinkage in the semi-solidified phase and in the surrounding solid metal shell but rather focuses its entire discussion on the minimum reduction rate on the rate of solidification shrinkage in the front end of the crater end within the strand in order to present a solution for the problem of center segregation and porosity. The fact is that it is not possible to eliminate the macro-segregations by only compensating for solidification shrinkage. Elimination of macro-segregations can only be effected by taking solidification shrinkage as well as cooling shrinkage in the solid and semi-solid phases into account. This was apparently not realized by Kawawa et al.
The method disclosed by Kawawa et al cannot be used on the strand just below the mold because the high reduction rate utilized would cause the molten metal to be pressed upward into the tundish since the solid shell of the strand constitutes only a small portion of the cross-sectional area. The Kawawa patent is premised on first forming a substantial solid shell on the strand during which time impurities are concentrated in the molten phase and to then press the concentrated molten steel backward from moving toward the front of the crater end. In this manner, a positive forcing of the concentrated molten steel backward along the strand length occurs as an integral part of the process.